Conventional varifocal objectives usually comprise an adjustable front group and a fixed rear group of coaxial lens members, the front group being constituted by an object-side first component considered stationary for the purpose of focal-length adjustment, axially shiftable second and third components, and an axially fixed fourth component. The first and fourth components are usually of positive refractivity whereas the shiftable second and third components are negatively refracting. The third component, sometimes termed compensator, moves in a manner designed to keep the image plane of the objective stationary during displacement of the second component or variator. Reference in this connection may be made, for example, to commonly owned U.S. Pat. No. 3,827,786 in the name of Karl Macher.
For focusing purposes, the quasi-stationary first component (or just a front lens thereof) is limitedly axially displaceable to an extent which does not significantly alter the selected overall focal length but shifts the rear focal plane of the objective relatively to the image plane. This axial displaceability of the first component or at least its front lens increases the physical axial length of the objective and also tends to cut off slanting incident rays, thus causing vignetting, unless the front lens is correspondingly enlarged. The presence of a large mobile front lens, however, makes the correction of aberrations more difficult; moreover, in a camera equipped with servofocusing the motor is burdened by the weight of this front lens. Finally, the air volume trapped behind the front lens within the camera housing impedes its movements unless fairly large vent openings are provided which, however, could make the optical system sensitive to atmospheric conditions.